C# Generics won't allow Delegate Type Constraints

Question

Is it possible to define a class in C# such that

class GenericCollection<T> : SomeBaseCollection<T> where T : Delegate

I couldn't for the life of me accomplish this last night in .NET 3.5. I tried using

delegate, Delegate, Action<T> and Func<T, T>

It seems to me that this should be allowable in some way. I'm trying to implement my own EventQueue.

I ended up just doing this [primitive approximation mind you].

internal delegate void DWork();

class EventQueue {
    private Queue<DWork> eventq;
}

But then I lose the ability to reuse the same definition for different types of functions.

Thoughts?

Solution 1

A number of classes are unavailable as generic contraints - Enum being another.

For delegates, the closest you can get is ": class", perhaps using reflection to check (for example, in the static constructor) that the T is a delegate:

static GenericCollection()
{
    if (!typeof(T).IsSubclassOf(typeof(Delegate)))
    {
        throw new InvalidOperationException(typeof(T).Name + " is not a delegate type");
    }
}

Solution 2

Yes it's possible in C# 7.3, Constraints family increased to include Enum, Delegate and unmanaged types. You can write this code without a problem:

void M<D, E, T>(D d, E e, T* t) where D : Delegate where E : Enum where T : unmanaged
    {

    }

From Docs:

Beginning with C# 7.3, you can use the unmanaged constraint to specify that the type parameter must be a non-nullable unmanaged type. The unmanaged constraint enables you to write reusable routines to work with types that can be manipulated as blocks of memory

Useful links:

The future of C#, from Microsoft Build 2018

What's new in C# 7.3?

Solution 3

Edit: Some proposed work-arounds are proposed in these articles:

http://jacobcarpenters.blogspot.com/2006/06/c-30-and-delegate-conversion.html

http://jacobcarpenters.blogspot.com/2006_11_01_archive.html

---

From the C# 2.0 specification we can read (20.7, Constraints):

A class-type constraint must satisfy the following rules:

• The type must be a class type.
• The type must not be sealed.
• The type must not be one of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.
• The type must not be object. Because all types derive from object, such a constraint would have no effect if it were permitted.
• At most one constraint for a given type parameter can be a class type.

And sure enough VS2008 spits out an error:

error CS0702: Constraint cannot be special class 'System.Delegate'

For info and investigation on this issue read here.

Solution 4

If you are willing to take a compile time dependency on an IL Weaver you can do this with Fody.

Using this addin to Fody https://github.com/Fody/ExtraConstraints

Your code can look like this

public class Sample
{
    public void MethodWithDelegateConstraint<[DelegateConstraint] T> ()
    {        
    }
    public void MethodWithEnumConstraint<[EnumConstraint] T>()
    {
    }
} 

And be compiled to this

public class Sample
{
    public void MethodWithDelegateConstraint<T>() where T: Delegate
    {
    }

    public void MethodWithEnumConstraint<T>() where T: struct, Enum
    {
    }
}

Solution 5

I came across a situation where I needed to deal with a Delegate internally but I wanted a generic constraint. Specifically, I wanted to add an event handler using reflection, but I wanted to use a generic argument for the delegate. The code below does NOT work, since "Handler" is a type variable, and the compiler won't cast Handler to Delegate:

public void AddHandler<Handler>(Control c, string eventName, Handler d) {
  c.GetType().GetEvent(eventName).AddEventHandler(c, (Delegate) d);
}

However, you can pass a function that does the conversion for you. convert takes a Handler argument and returns a Delegate:

public void AddHandler<Handler>(Control c, string eventName, 
                  Func<Delegate, Handler> convert, Handler d) {
      c.GetType().GetEvent(eventName).AddEventHandler(c, convert(d));
}

Now the compiler is happy. Calling the method is easy. For example, attaching to the KeyPress event on a Windows Forms control:

AddHandler<KeyEventHandler>(someControl, 
           "KeyPress", 
           (h) => (KeyEventHandler) h,
           SomeControl_KeyPress);

where SomeControl_KeyPress is the event target. The key is the converter lambda - it does no work, but it convinces the compiler you gave it a valid delegate.

(Begin 280Z28) @Justin: Why not use this?

public void AddHandler<Handler>(Control c, string eventName, Handler d) { 
  c.GetType().GetEvent(eventName).AddEventHandler(c, d as Delegate); 
} 

(End 280Z28)

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Comments

[Nicholas Mancuso]

thats not really the functionality i'm looking for... I was trying to make a type constraint on my generic class...

[Sam Harwell]

@Justin: I edited your answer to put my comment at the end since it has a code block.

[Sam Harwell]

+1 for: 1) using the static constructor and 2) including a detailed message due to weird debugging conditions surrounding type initialization.

[myermian]

@MarcGravell: Doesn't throwing an exception in a static initializer violate CA1065: Do not raise exceptions in unexpected locations ... I was always under the assumption that you should use a custom code analysis rule to find invalid usages of your class that aren't normally available at run-time.

[Justin Morgan]

Broken link. Do you have a current one?

[Jeppe Stig Nielsen]

Yes, it is possible in C# 7.3 (since May 2018), and you can see release notes here.

[Jeppe Stig Nielsen]

Starting in C# 7.3 (released May 2018), it is allowed to constrain like this, where T : Delegate, (and someone posted a new answer about that below).

[Elmue]

Why do you repeat the question here? You don't tell us anything new.

[Joel]

This should be the new accepted answer, the current one is from 2008.. Very outdated now.